1. Field of the Invention
The present invention relates to a second-harmonic generation device having a semiconductor laser element and a wavelength conversion element which converts the wavelength of light emitted from the semiconductor laser element to a half wavelength. In addition, the present invention relates to a semiconductor laser element, and particularly to a semiconductor laser element having an InGaP cladding layer above a GaAs substrate.
2. Description of the Related Art
(1) Laser devices which are constituted by a semiconductor laser element and a wavelength conversion element and emit light having a blue or green wavelength have been developed, where the oscillation wavelength of the semiconductor laser element is in the 0.9 to 1.2 μm band, and the wavelength conversion element promotes generation of a second harmonic. For example, in some of the above laser devices, the wavelength conversion element has a waveguide structure made of LiNbO3, the semiconductor laser element is directly coupled to the wavelength conversion element, and single-wavelength laser light emitted from the semiconductor laser element and matched with the wavelength conversion element is supplied to the wavelength conversion element. Thus, the wavelength of the laser light from the semiconductor laser element is converted to a half wavelength. However, the wavelength conversion element is very sensitive to the wavelength of the fundamental laser light, and the conversion efficiency greatly varies with a slight variation in the wavelength. Therefore, the intensity of the laser light after the wavelength conversion is likely to become unstable. Thus, when a semiconductor laser element is used in a fundamental-light source, the oscillation characteristics are required to be highly stable.
In consideration of the above problem, conventionally, distributed Bragg reflection-type lasers (DBR-LDs) or Fabry-Perot (FP) lasers being combined with a wavelength filter and having an arbitrarily controlled oscillation wavelength are mainly used as fundamental-light sources using a semiconductor laser element. The primary objective in the fundamental-light sources using the Fabry-Perot (FP) lasers is to stably control the oscillation wavelength, and various research organizations are currently making an attempt to stabilize the oscillation wavelength of the semiconductor laser element. In one of such an attempt, only a portion within a narrow line width of light emitted from the semiconductor laser element is returned by using a diffraction grating or etalon so as to stabilize the wavelength and enhance coherence. When this technique is used, it is possible to overcome the problem of the semiconductor laser element that the wavelength is likely to vary. Thus, the above technique is effective in the fundamental-light sources which are used with a wavelength conversion element.
In the case where a semiconductor laser element is used for generating a fundamental harmonic in a second-harmonic generation device, the peak gain wavelength of the semiconductor laser element is important. The gain peak in a wavelength spectrum of a semiconductor laser element has a certain width. When the width of the gain peak becomes smaller and the peak height becomes higher, the threshold becomes lower and the characteristics are improved. However, in the case where a semiconductor laser element having a small gain peak width is used as a light source, and the wavelength of the semiconductor laser element is controlled by external feedback of light, a width within which the wavelength can be varied becomes small. Therefore, when the gain peak width is small, and the difference between the peak gain wavelength of the semiconductor laser element and a control wavelength of the external light is increased, in some cases, it becomes impossible to fix the wavelength to a desired value.
In particular, the widths of gain peaks of semiconductor laser elements having a quantum-well active layer are smaller than those of the semiconductor laser elements having a conventional bulk active layer. Further, in the case of semiconductor laser elements which emit laser light in the wavelength range of 0.9 to 1.2 micrometers, the wavelengths in the wavelength range cannot be generated without a quantum-well structure, the gains are high, and the widths of gain peaks are small. Therefore, it is difficult to control the wavelengths in the wavelength range of 0.9 to 1.2 micrometers by using an external optical filter such as a diffraction grating.
(2) Since semiconductor laser elements are useful due to their small size, low price, high efficiency, low power consumption, and the like, the semiconductor laser elements are recently receiving attention in various fields, and are widely used, in particular, as light sources. Almost all of the currently available semiconductor laser elements are produced by forming fundamental layers including cladding layers, an active layer, a current confinement layer, a contact layer, and the like on a GaAs or InGaP substrate by crystal growth, and making a structure for mode control, current confinement, and the like through semiconductor processes including a lithography process, a machining process, and the like.
However, the material dependence of the semiconductor processes is high. In particular, shapes formed by wet etching processes vary depending on materials and crystal orientations. Therefore, when some materials are used, it is difficult to realize a desired shape.
As an example of the materials which are difficult to be processed is AlGaAs. For example, a semiconductor laser element in the 0.8 μm band is produced through the following processes. First, an n-type AlGaAs cladding layer, an active layer, a p-type AlGaAs cladding layer, and a p-type GaAs contact layer are formed on an GaAs substrate, and thereafter a ridge is formed by forming a mask on the layered structure and etching disclosed portions of the layered structure to a mid-thickness of the p-type AlGaAs cladding layer. In this case, since the width of the ridge is a very important parameter for controlling the semiconductor laser element in a transverse mode, the ridge is required to be precisely formed.
When the p-type AlGaAs cladding layer is etched for formation of the ridge, it is difficult to control the ridge shape since unignorable portions of the layered structure under the mask are etched in the lateral directions (i.e., unignorable side etching occurs). Therefore, in order to realize the above structure, it is necessary to control the mask width, the etching time, the temperature of an etching solution, and the like with high accuracy. In addition, even when these factors are accurately controlled, the ridge shapes formed in an actual manufacturing system vary depending on positions within each wafer surface. Thus, the yield rate becomes low, and the cost increases.
(3) The erbium-doped fiber amplifiers (EDFA) have been developed in the 1990s, and recently the communication capacities in wavelength-division-multiplex communications have been increasing. In this situation, the semiconductor lasers in the 0.98 μm band, which are used as excitation light sources in the EDFAs, are expected to have high optical output power and reliability.
In addition, high-output-power semiconductor lasers in the 1.02 μm and 1.05 μm bands are expected for use as excitation light sources in other fiber amplifiers.
Further, laser devices constituted by a semiconductor laser having an oscillation wavelength in the 0.9 to 1.1 μm band and a polarization-inverted-domain distribution element promoting generation of a second harmonic have been developed, where the laser devices emit light having a blue or green wavelength.
Thus, currently, various manufacturers are pursuing development of semiconductor lasers in the wavelength range of 0.9 to 1.1 μm, as disclosed in, for example, Japanese Unexamined Patent Publication, Nos. 06(1994)-077588, 05(1993)-275801, and 05(1993)-037078, and U.S. Pat. No. 5,530,713, and the like.
Although the above Japanese Unexamined Patent Publications mainly disclose improvements in layer structures and shapes of the semiconductor laser elements, the disclosed semiconductor laser elements are not optimized with respect to impurity doping concentrations. The impurity doping concentration is an important parameter for determining optical output power characteristics of semiconductor laser elements. In particular, when concentrations of p-type dopants are too high, optical loss and defects increase. On the other hand, when the concentrations of p-type dopants are too low, the barrier functions against electrons become weak, and therefore overflow is likely to occur during high output power operation. Normally, zinc (Zn) is used as a p-type dopant in a p-type InGaP cladding layer. However, since the mobility of Zn ions in InGaP is great, the Zn ions diffuse into an active layer, and become non-radiative recombination centers, which increase loss in a resonator and cause crystal defects.